Optical characterization of CdS nanoparticles embedded into the comb-type amphiphilic graft copolymer
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- Kalaycı, Ö.A., Duygulu, Ö. & Hazer, B. J Nanopart Res (2013) 15: 1355. doi:10.1007/s11051-012-1355-x
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This study refers to the synthesis and characterization of a novel organic/inorganic hybrid nanocomposite material containing cadmium sulfide (CdS) nanoparticles. For this purpose, a series of polypropylene (PP)-g-polyethylene glycol (PEG), PP-g-PEG comb-type amphiphilic graft copolymers were synthesized. PEGs with Mn = 400, 2000, 3350, and 8000 Da were used and the graft copolymers obtained were coded as PPEG400, PPEG2000, PPEG3350, and PPEG8000. CdS nanoparticles were formed in tetrahydrofuran solution of PP-g-PEG amphiphilic comb-type copolymer by the reaction between aqueous solutions of Na2S and Cd(CH3COO)2 simultaneously. Micelle formation of PPEG2000 comb-type amphiphilic graft copolymer in both solvent/non-solvent (petroleum ether–THF) by transmission electron microscopy (TEM). The optical characteristics, size morphology, phase analysis, and dispersion of CdS nanoparticles embedded in PPEG400, PPEG2000, PPEG3350, and PPEG8000 comb-type amphiphilic graft copolymer micelles were determined by high resolution TEM (HRTEM), energy dispersive spectroscopy, UV–vis spectroscopy, and fluorescence emission spectroscopy techniques. The aggregate size of PPEG2000-CdS is between 10 and 50 nm; however, in the case of PPEG400-CdS, PPEG3350-CdS, and PPEG8000-CdS samples, it is up to approximately 100 nm. The size of CdS quantum dots in the aggregates for PPEG2000 and PPEG8000 samples was observed as 5 nm by HRTEM analysis, and this result was also supported by UV–vis absorbance spectra and fluorescence emission spectra.
KeywordsQuantum dotsCadmium sulfideComb-type amphiphilic copolymerMicelleHRTEM
The semiconductor nanoparticles below 10 nm, which are also named as quantum dots, have recently become technologically important (Kittel 1986; Heinglein 1989). The quantum dots gain unique electronic, optical, magnetic, and mechanical properties, as the electronic energy states of these materials become discrete. When the size and shape of quantum dots change, the properties related with them also change, compared to their properties observed in the bulk and atomic/molecular levels (Biju et al. 2008). As the particle size of the quantum dots increases, the emission spectrum is observed with a low energy region. On the other hand, when the particle size of the quantum dots decreases, the emission spectrum is observed with a higher energy region (Tamborra et al. 2004; Brush 1986; Wang and Blau 2009; Tang et al. 2005; Kumbhakar et al. 2008; Talapin et al. 2010; Chestnoy et al. 1986). The optical properties of the quantum dots can be studied by UV–vis absorbance spectrum, photoluminescence, electroluminescence, and Raman dispersion (Trindade et al. 2001; Alivisatos 1996; Pesika et al. 2003).
A direct band gap material, II–VI semiconductor cadmium sulfide (CdS), can be used in the production of photoelectronic devices. In optical switches, sensors, electroluminescent devices, lasers, and biomedical tags, there are potentially possible applications for luminescence of the CdS quantum dots (Tessler et al. 2002). In nanometer scale, polymers, colloids, micelles, thin films, and zeolites are used to synthesize the nanocomposite materials containing quantum dots like CdS or metal particles (Pomogailo 2000; Farmer and Patent 2001; Nabok 2005; Tomczak et al. 2009; Tura et al. 2005; Thakur et al. 2009; Dong and Hinestroza 2009; Hazer et al. 2012a, b; Hazer and Hazer 2011; Nicolais and Carotenuto 2005). The synthesis and characterization of the polypropylene (PP)-g-polyethylene glycol (PEG) amphiphilic comb-type copolymers have been recently reported in our study (Balcı et al. 2010). The PP-g-PEG amphiphilic comb-type copolymer containing metal nanoparticles were also observed in terms of the antibacterial impact (Kalaycı et al. 2010) and the in vivo biocompatibility (Hazer et al. 2011, 2012a, b). So as to obtain nanoparticles in polymeric matrix, various methods are available such as the chemical reduction, photoreductions, thermal decompositions, and vapor deposition methods. Stabilized colloidal sols or thin film after the evaporation solution can be obtained, depending on the method. The host polymer not only acts as protective polymer to form a dispersing and stabilizing media for the nanoparticles but also acts as functional polymer to affect the material and particle properties directly by surrounding nanoparticles (Mayer 2001).
Amphiphilic copolymers, including hydrophobic and hydrophilic blocks are known as suitable “steric stabilizator” for the nanoparticles. The mutual interaction of polymer blocks and particles in a hybrid structure can affect the dispersion, location and size of the particles (Chiu et al. 2005; Oren et al. 2009; Kim et al. 2006). The amphiphilic copolymers containing PEG as hydrophilic segment especially have specific importance because of their high ratio in self-assembly, tendency in phase formation and biocompatibility (Yıldız et al. 2012; Erciyes et al. 1992; Hazer 1995, 2010).
In the solvent or surface medium, the hydrophilic and hydrophobic blocks stick on each other and form micelle, micro emulsion, and adsorbed polymer layers. Since one block settles down relatively in a solvent environment, the copolymer chains form micelle clusters and they take the form of colloidal distribution (Mayer 1998). In the amphiphilic copolymers, micelle formation depends on the pair of the solvent/non-solvent. In the micelle formation, non-soluble block settles down in one micelle core, while the soluble block disperses around the core.
Many studies in literature have presented various ways of obtaining nanoparticles like CdS within polymer matrix (Lemon and Crooks 2000; Liu et al. 2003; Herron et al. 1990; Jamali et al. 2007; Spanhel et al. 1987; Underhill and Liu 2000; Firth et al. 2004; Peretz et al. 2011; Ferrer et al. 2009; Yeh et al. 2003; Pandey and Pandey 2009; Moore and Patel 2001). Zhao et al. have studied PS-b-P2VP micelles that involve corona-embedded CdS nanoparticles (Zhao and Douglas 2002; Zhao et al. 2001). Moffitt et al. (1998) have obtained core-embedded CdS quantum dots within polystyrene-b-poly(cadmium acrylate) (PS-b-PACd) reverse micelles. Petit et al. (1990) have obtained very small CdS nanoparticles in reverse micelles by in situ synthesis. Qi et al. (2000) have analyzed high stabilized CdS nanoparticles within PEG block and poly(ethylene imine) double-hydrophilic block copolymers.
In this study, which is one of the studies of our research group, the preparation of a new composite material which stabilizes the CdS quantum dots into the PP-g-PEG comb-type amphiphilic graft copolymers has been reported (Balcı et al. 2010; Kalaycı et al. 2010; Hazer et al. 2011, 2012a, b). The CdS quantum dots into this polymer matrix were studied in view of the micelle formation by high resolution transmission electron microscopy (HRTEM) and its optical behavior by UV–vis spectroscopy and fluorescence emission spectroscopy techniques.
Chlorinated polypropylene (PP-Cl, Mw = 150,000 Da, three repeating units have 1 Cl in average), PEG Mn = 400, 2000, 3350, 8000 Da and Cd(CH3COO)2 were supplied from Sigma-Aldrich. Tetrahydrofuran was supplied from Sigma-Aldrich and it was purified before being used.
The synthesis of PP-g-PEG comb-type amphiphilic graft copolymer
The PP-g-PEG block copolymer synthesis was conducted by keeping the PEG ratio at 10 %, as defined in the procedure presented in our previous studies (Balcı et al. 2010; Kalaycı et al. 2010). A typical endcapping reaction was performed as the following: PEG-2000 (5.0 g, 2.5 mmol) and PP-Cl (1.43 g, 1.0 mmol Cl) were separately dissolved in dry THF (30 mL). NaH in oil (60 wt%) (0.20 g, 5 mmol NaH) was added to the PEG solution and the reaction mixture was stirred at room temperature under argon atmosphere for 3 h. Sodium salt of PEG solution was added to the PP-Cl solution and stirred for half an hour. The reaction mixture was stirred one more day and poured into 500 mL of 1 M HCl. The precipitated polymer was filtered, washed with distilled water, and dried under vacuum at 50 °C for a day. For the purification, it was redissolved in chloroform and reprecipitated in 200 mL of methanol, and dried under vacuum overnight at room temperature. The yield was 1.4 g. In order to observe the micelle formation of amphiphilic graft copolymer, pure water was added by continuously stirring the 5 mL THF solution of 0.1 g of the polymer until the turbidity was observed.
The preparation of the PP-g-PEG amphiphilic graft copolymers including CdS nanoparticles
One hundred milligrams of PP-g-PEG was dissolved in 10 mL of THF, and 0.05 mL of 0.2 M Cd(CH3COO)2 aqueous solution was poured on it and the solution was stirred for half an hour. Then, 0.05 mL of 0.3 M Na2S aqueous solution was added to this solution. The yellow color of CdS was observed within 2–3 min. Following this step, the solution was stirred for 1 h, and poured into a Petri dish (diameter, ϕ = 7 cm) by means of film formation via solvent casting. After 1 day, composite polymer film was peeled away from the Petri dish. The composite yellow film was washed with methanol and dried under vacuum at room temperature for a day. The composite polymer was dissolved in 20 mL of toluene. The toluene solution was filtered and used to observe the optical characteristics.
The sample preparation and instrumentation
The solutions were put in ultrasonic bath for 10 min. The TEM sample preparation was done by placing a solution on 200-mesh carbon-coated copper TEM grid (EMS CF200-Cu). The TEM measurements were conducted using JEOL JEM-2100 HRTEM at 200 kV (LaB6 filament) and the elemental analysis were carried on via Oxford Instrument with 6498 energy dispersive spectroscopy (EDS) system. The images were recorded using slow scanning CCD camera (Gatan-694 model) with Gatan Digital Micrograph software. The low magnification TEM imaging, selected area electron diffraction (SAED), HRTEM (atomic lattice imaging and FFT diffractograms), and EDS techniques were used.
The UV–vis absorption spectra of the colloidal solutions of nanoparticle-embedded polymer prepared with the toluene were measured at room temperature using Cary 100 model UV–vis spectrometer. The fluorescence emission spectra measurements were done using Cary Eclipse model Fluorescence Spectrometer instrument under the 350-nm wavelength at room temperature.
Results and discussion
The examination of micelle formation of PP-g-PEG amphiphilic graft copolymer
TEM analysis of the micelle formation of CdS nanoparticles embedded in the pp-g-peg amphiphilic graft copolymer
The low magnification TEM images illustrate the relatively good size and morphology distribution of the particles in the polymer matrix. The aggregate is observed for all samples. It is seen that the aggregated particle size is the smallest in the PPEG2000-CdS sample. The aggregate size of PPEG2000-CdS is between 10 and 50 nm; however, in the case of PPEG8000-CdS, PPEG3350-CdS, and PPEG400-CdS samples, the aggregate size is up to approximately 100 nm (Fig. 6).
Figure 7 shows the representative TEM–EDS spectrum and elemental composition of CdS in PPEG2000 micelle, which confirms the Cd and S coexistence in the micelle. Moreover, Cd to S stoichiometric ratio is found as nearly 1:1. A relatively small amount of Cl is also observed. C and O elements emerge from the structure of polymer matrix and C, Cu and Si elements are from the TEM grid.
Furthermore, the SAED pattern (Fig. 8) of CdS embedded PPEG3350-CdS revealed d-spacings of 3.37 Å (A), 2.06 Å (B), 1.76 Å (C) and 1.30 Å (D) which corresponded to (111), (220), (311), and (420) lattice planes of cubic CdS phase, respectively.
The quantum dots size around 5 nm was measured for specimens of PPEG2000-CdS and PPEG8000-CdS. The nanoparticles were as mostly spherical. Figures 9 and 10 show HRTEM images and FFT diffractograms of CdS quantum dots in two different regions of PPEG2000-CdS sample. Similarly, Fig. 11 depicts HRTEM images and FFT diffractograms of CdS quantum dots in PPEG8000-CdS sample.
Figures 9d, 10d, and 11d illustrate FFT diffractograms for CdS quantum dots. In the FFT diffractograms, the inner circle spots and the arrows represented d-spacings of 3.37 and 2.06 Å, respectively. Similarly, these d-spacings were also measured on the HRTEM atomic lattice images (Figs. 9c, 10c, e, 11c).
Therefore, HRTEM images and FFT micrographs exhibited lattice fringes with interplanar distances of 3.37 and 2.06 Å which are d-spacings for (111) plane and (220) planes of CdS cubic phase (ICCD pdf number: 89-0440, space group: 216), respectively. Since other CdS phase (hexagonal (ICCD pdf number: 80-0006, space group: 186) or orthorhombic (ICCD pdf number: 47-1179, space group: 48) lattice spacings were not observed, it can be stated that the particles are mainly cubic CdS phase.
The atomic lattice imaging could be performed on PPEG2000-CdS and PPEG8000-CdS samples. However, HRTEM technique was not successful for PPEG400-CdS and PPEG3350-CdS specimens. Yet, TEM imaging could be achieved (Fig. 12). This can be related to the artifacts of the TEM sample preparation. Since a week has passed between the TEM sample preparation and the solution preparation, it is thought that the PEG chains were destroyed and the stability of the solution was deformed.
The solution characteristics and UV–vis analysis
In a semiconductor, the band gap between the valance and conduction band increases with the decreases, in the particle size. If the size of semiconductor nanoparticle is less than Bohr radius, the quantum confined can be considered as a exciton. The excitation of electron from valance band to conduction band depends on the energy that is higher than the band gap. Thus, UV–vis absorbance spectrum is observed in a lower wavelength region. This absorption band exists because of the first optically allowed transition of CdS between the conduction band and hole state in valance band. As the interaction of excitons of the small size nanocrystals in the aggregates results in the weak and slowly band transition, no observed for the sharp and strong band transition when the UV–vis absorbance spectrum of PPEG8000 sample is analyzed.
The absorbance wavelengths were detected as 460, 480, 462, and 482 nm for the absorbance spectra curve of PPEG400-CdS, PPEG2000-CdS, PPEG3350-CdS, and PPEG8000-CdS, respectively. It was observed that the wavelength value of the CdS bulk shifted from 515 to 460 (482, 480, and 462) nm. This situation is only expected when the particle size is below 10 nm . This result matches with the HRTEM images, as for PPEG2000-CdS and PPEG8000-CdS samples, CdS nanoparticles smaller than 10 nm were detected. The CdS quantum dots were not as single and separate; but, observed as attached to each other in aggregations.
The solution characteristics and fluorescence emission spectra
The broad emission peak for PPEG400-CdS, PPEG2000-CdS, and PPEG3350-CdS was observed at 625 nm and for PPEG8000 at 640 nm in emission spectra. In addition, the peaks available in PPEG400-CdS, PPEG2000-CdS, PPEG3350-CdS, and PPEG8000-CdS were observed at 445, 460, 445, and 417 nm, respectively. The broad peak, observed in emission spectra, is closely related to the size of CdS nanoparticles and surface state. In literature, the broad peak, caused by the surface state, is determined between 530 and 650 nm (Cao et al. 2005). The peak in the lower energy region can be caused by the nanoaggregations behaving like big size CdS nanoparticles. The peaks between 420 and 460 nm correspond to the band edge emission of the CdS nanoparticles. From HRTEM images, nanoaggregations, having 5-nm size CdS nanocrystals in the hybrid structures have been determined. As a result, the colloidal solution includes both monocrystalline CdS (below 10 nm) and CdS aggregation.
In a water/polar medium, Cd2+ ion cores settle down the corona region of micelles by interacting with non-ionic hydrophilic PEG chains. With the addition of Na2S, Cd2+ ion cores and S2− interact, and thus CdS nanoparticles are obtained. CdS nanoparticles, settling in the regions that Cd2+ ions cores select, are among PEG chains. During this process, PP chains form the core of micelle by aggregating. Controlling the procedure during the transition from Cd2+ to CdS in the solvent can change the particle features. It is essential to provide the stability of solution during the transition from Cd2+ to CdS and thus goal of this study, that is to obtain colloidal solution stabilized within CdS nanoparticles, has been achieved.
As a result, by means of strong film formation, the PP-g-PEG comb-type amphiphilic copolymers with good mechanical properties stabilized the CdS nanoparticles as successful as they did the metal nanoparticles. The PEG hydrophilic chains form stable bonds with the semiconductor nanoparticles preserving the characteristics of these materials. According to the HRTEM images, CdS quantum dots in the polymer micelles were below 10 nm size and this was supported by their optical characterization. The results show that PP-g-PEG amphiphilic comb-type graft copolymer is a fine colloidal stabilizer and host for nanosized particle formation. Therefore, the organic–inorganic PP-g-PEG amphiphilic comb-type graft copolymer CdS quantum dot composite material can be regarded as the promising material for the industrial applications because of its strong film forming qualification.
This work was supported by; both the Bulent Ecevit University Research Fund (#BEU-2012-10-03-13) and TUBITAK (Grant # 211T016).